In the wavelength division multiplexed (WDM, hereinafter) optical communication, optical signals propagated through an optical transmission line (optical fibers) respectively undergo wavelength dispersions The wavelength dispersion is a kind of dispersion, and means that a shape of an optical pulse propagated through the optical fiber is deformed or broadened in various ways depending on a slight difference in the wavelength of the optical pulse, and group velocities of the respective optical signals take different values, chiefly because the refractive index of the optical fiber varies as a function of the wavelength of a light.
Accordingly, the wavelength dispersion is a primary factor of a limitation of the transmission distance of the optical communication system, or of a deterioration of the quality of the optical transmission caused by the distortion of the optical pulse. Especially, in the long distance transmission system using Erbium-doped fiber amplifiers, since the optical signal is propagated through the optical fiber without being converted into an electrical signal from the sending end to the receiving end, the wavelength dispersion caused in the total length of the transmission line is accumulated on the optical signal.
Moreover, in a case of the long distance transmission system such as the submarine optical transmission system or of a high bit rate transmission system, the effect of the accumulation of the wavelength dispersion becomes a serious problem. Accordingly, the effect of the wavelength dispersion must be compensated by taking some measures, and explaining concretely, the wavelength dispersion has been thus far compensated by inserting the dispersion compensation fiber into the optical transmission line, where the wavelength dispersion of the dispersion compensation fiber should have the same absolute value as and a sign opposite to that of the optical transmission line.
The structure of the optical transmission line using the dispersion compensation fiber is shown in FIG. 1. In general, the wavelength dispersion of the dispersion compensation fiber 110 and that of the optical fiber for the signal transmission 120 change in accordance with the wavelength of the optical signal. Accordingly, if the wavelength dispersion of the dispersion compensation fiber 11 has the same absolute value as and a sign opposite to that of the optical fiber for the signal transmission 120 independently of the wavelength, the wavelength dispersion of the optical signal must be compensated perfectly at any wavelength.
However, although it is possible to theoretically cancel the wavelength dispersion of the optical fiber for the signal transmission 120 by the wavelength dispersion compensation fiber 110 having the aforementioned characteristic, it has been impossible to actually provide the wavelength dispersion compensation fiber 110 which cancels the wavelength dispersions of the optical fiber for the signal transmission throughout all the channels of the WDM optical signals.
Herein, FIG. 2 shows a dispersion map of the optical signals in case that the dispersion compensation fibers having the positive wavelength dispersion are inserted into the optical transmission line composed of optical fibers for the signal transmission having the negative wavelength dispersion. As shown in FIG. 2, the wavelength dispersions of the optical signal λ1 to λ5 to are compensated by the dispersion compensation fibers, and approach zero at a certain interval of the transmission distance.
As seen from FIG. 2, although only the wavelength dispersion of the optical signal λ3 return to zero whenever it is compensated by the dispersion compensation fibers, those of the other optical signals λ1, λ2, λ4, λ5, do not return to zero even when they are compensated by the same, because the wavelength dispersions of the optical signals vary as functions of the wavelengths of the optical signals, and thereby the wavelength dispersions are accumulated on the optical signals depending on the transmission distance.
That is to say, although the conventional dispersion compensation fiber can compensate the accumulated wavelength dispersion satisfactory only when it is limited within a certain value, it cannot return the accumulated wavelength dispersion exceeding the certain limit to zero. Namely, the range of the accumulated wavelength dispersion which can be satisfactorily compensated by the conventional dispersion compensation fiber is limited.
Moreover, following disadvantages are pointed out on the conventional dispersion compensation fiber. That is to say, an insertion loss of the conventional dispersion compensation fiber becomes large as the wavelength dispersion to be compensated becomes large. The dispersion compensation fiber necessitates a length which is proportional to the length of the optical transmission line, and the weight thereof becomes large. Accordingly, it becomes difficult to make the dispersion compensator using the conventional wavelength dispersion compensation fiber and devices concerned therewith compact and lightweight, and to reduce consumed electric power and the cost price thereof.
Although it can be considered that the accumulated wavelength dispersions are compensated in the lump at the transmitting and receiving terminals (not shown), since the accumulated wavelength dispersion exceeding a certain value cannot be compensated similarly to the conventional dispersion compensation fiber, it is impossible to compensate the accumulated wavelength dispersions throughout all the cannels of the WDM optical signals.